Universal ISDN/POTS splitter

ABSTRACT

A 7 th  order splitter for replacing the known 9 th  order splitter of an ISDN/POTS telecommunication system and reducing thereby the space occupied on the board as well as the cost. The 7 th  order splitter comprising, between line terminals (Line+, Line−) and POTS/ISDN terminals, the cascade connection of a LC filter cell (L 21   a , L 12   b , C 21 ) and several elliptical filter cells (L 22   a , L 22   b , C 22 , C 24 , C 25 ; L 23   a , L 23   b , C 23 , C 26 , C 27 ). One or more of these filter cells is associated with a damped series resonance circuit comprising a capacitor (C 21 ; C 22 ; C 23 ) series coupled with an RL circuit (R 21 , L 24 ; R 22 , L 25 ; R 23 , L 26 ) between two winding (L 21   a , L 21   b ; L 22   a , L 22   b ; L 23   a , L 23   b ) of a symmetrical coil of the associated filter cell. Each RL circuit is constituted by the parallel coupling of a resistor (R 21 ; R 22 ; R 23 ) and a coil (L 24 ; L 25 ; L 26 ).

The present invention relates to a splitter for an ISDN/POTStelecommunication system, said splitter having first and second lineterminals and first and second POTS/ISDN terminals, and comprising thecascade connection of a LC filter cell and a plurality of ellipticalfilter cells, said LC filter cell comprising a first symmetrical coilhaving a first winding with a first terminal connected to said firstline terminal and with a second terminal coupled to said first POTS/ISDNterminal via said plurality of elliptical filter cells, and having asecond winding with a first terminal connected to said second lineterminal and with a second terminal coupled to said second POTS/ISDNterminal via said plurality of elliptical filter cells, said LC filtercell further comprising a first capacitor coupled between the secondterminal of the first winding of said first symmetrical coil and thesecond terminal of the second winding of said first symmetrical coil,each elliptical filter cell of said plurality comprising a secondsymmetrical coil having a first winding with a first terminal coupled tothe second terminal of the first winding of said LC filter cell and witha second terminal coupled to said first POTS/ISDN terminal and having asecond winding with a first terminal coupled to the second terminal ofthe second winding of said LC filter cell and with a second terminalcoupled to said second POTS/ISDN terminal, each elliptical filter cellfurther comprising a second capacitor coupled between the secondterminal of the first winding of the second symmetrical coil of saidelliptical filter cell and the second terminal of the second winding ofsaid second symmetrical coil, a third capacitor connected across thefirst winding of said second symmetrical coil and a fourth capacitorconnected across the second winding of said second symmetrical coil.

Today's ISDN and Universal ISDN/POTS splitter circuits for xDSL aregenerally known in the art. These known splitters are based on 9^(th)order low pass filters, consisting of 4 coils and the related highvoltage capacitors (typically 10 capacitors) as shown at FIG. 1.

This amount of large coils and capacitors per DSL line limits theintegration of the number of splitter circuits per board (e.g. 72 portsper splitter board) and prevent the integration of a high number of DSLline terminations and splitter lines on a single board. As the number ofcoils and capacitors is high, not only the required space but also thecost of the known 9^(th) order ISDN splitter is relatively high.

Moreover, the known Universal ISDN/POTS splitter low pass filters haveto be standard compliant (ETSI TS 101 952-1-4). In most cases, twodifferent filters with a same topology but different component valuesare required to cover the 2 ISDN standards: 2B1Q and 4B3T.

An object of the present invention is to provide a splitter of the aboveknown type but whereof the size and the cost are dramatically reducedwhile the splitter characteristics remain compliant to the ETSI standardETSI TS 101 952-1-4, for 2B1Q and 4B3T ISDN standards.

According to the invention, this object is achieved due to the fact thatsaid splitter comprises at least one RL circuit constituted by theparallel coupling of a resistor and a coil and connected in series withthe second capacitor of an elliptical filter cell of said plurality ofelliptical cells.

In this way, a 7^(th) order ISDN or Universal ISDN/POTS splitter lowpass filter is provided for replacing the known 9^(th) order low passfilter. By eliminating 1 elliptical cell compared to the 9^(th) ordersplitter low pass filter, and by adding at least one RL circuitassociated to another elliptical filter cell, a size reduction of 25%and a significant cost reduction is achieved.

This 7^(th) order filter is full ETSI standard compliant (ETSI TS 101952-1-4) for both 2B1Q and 4B3T ISDN standards. The present splitterfurther shows comparable dynamic performance compared to the 9^(th)order splitter (transient rejection, noise impact on DSL signals, noiseimpact on ISDN signals).

It can further be proved that, with the present splitter, the xDSLstop-band attenuation characteristic, the ISDN pass-band return loss andinsertion characteristic, both with and without ADSL impedance (Zadsl),and the POTS pass-band return loss and insertion loss characteristicsfor different impedances and conditions are similar to those of theknown splitter.

Another characterizing embodiment of the present invention is that saidsplitter comprises another RL circuit constituted by the parallelcoupling of a second resistor and a second coil and connected in serieswith the first capacitor of said LC filter cell between the secondterminal of the first winding of said first symmetrical coil of said LCfilter and the second terminal of the second winding of said firstsymmetrical coil of said LC filter.

By providing a RL circuit in the LC filter cell in addition to the oneor more RL circuits associated to the elliptical filter cells, theperformances of the splitter are increased.

In a preferred characterizing embodiment of the present invention, saidsplitter comprises the series connection of said LC filter cell, a firstelliptical filter cell comprising a first second capacitor and a secondelliptical filter cell comprising a second capacitor, the first secondcapacitor of said first elliptical filter cell is connected in serieswith a first RL circuit constituted by the parallel coupling of a firstresistor and a first coil between the second terminal of the firstwinding of the second symmetrical coil of said first elliptical filtercell and the second terminal of the second winding of the secondsymmetrical coil of said first elliptical filter cell, and the secondcapacitor of said second elliptical filter cell is connected in serieswith a second RL circuit constituted by the parallel coupling of asecond resistor and a second coil between the second terminal of thefirst winding of the second symmetrical coil of said second ellipticalfilter cell and the second terminal of the second winding of the secondsymmetrical coil of said second elliptical filter cell.

This arrangement constitutes the optimal splitter with respect to size,cost and standard compliance.

Further characterizing embodiments of the present a splitter for anISDN/POTS telecommunication system are mentioned in the appended claims.

It is to be noticed that the term ‘comprising’, used in the claims,should not be interpreted as being restricted to the means listedthereafter. Thus, the scope of the expression ‘a device comprising meansA and B’ should not be limited to devices consisting only of componentsA and B. It means that with respect to the present invention, the onlyrelevant components of the device are A and B.

Similarly, it is to be noticed that the term ‘coupled’, also used in theclaims, should not be interpreted as being restricted to directconnections only. Thus, the scope of the expression ‘a device A coupledto a device B’ should not be limited to devices or systems wherein anoutput of device A is directly connected to an input of device B. Itmeans that there exists a path between an output of A and an input of Bwhich may be a path including other devices or means.

The above and other objects and features of the invention will becomemore apparent and the invention itself will be best understood byreferring to the following description of an embodiment taken inconjunction with the accompanying drawings wherein:

FIG. 1 shows a splitter for an ISDN/POTS telecommunication system asknown from the prior art;

FIG. 2 shows a splitter according to the invention;

FIG. 3 shows a variant of the splitter of FIG. 1; and

FIG. 4 shows a comparison between the space occupied by a prior artsplitter with respect to the space occupied by the splitter of thepresent invention on a board.

A splitter for an ISDN/POTS telecommunication system or moreparticularly the 9^(th) order low pass filter thereof as known from theprior art is shown at FIG. 1. This splitter has filter stages locatedbetween line terminals and POTS/ISDN terminals. An additional CommonMode Coil and protector for overvoltages may also be provided but arenot shown at FIG. 1.

In more detail, the known splitter has a first line terminal Line+, asecond line terminal Line−, a first POTS/ISDN terminal POTS/ISDN+ and asecond POTS/ISDN terminal POTS/ISDN−. Between these line and POTS/ISDNterminals, the splitter comprises the cascade connection of a LC filterstage or cell Lila, L11 b, C11 and three elliptical filter stages L12 a,L12 b, C12, C15, C16; L13 a, L13 b, C13, C17, C18; L14 a, L14 b, C14,C19, C1A, R11, R12.

The LC filter cell comprises a first symmetrical coil L11 constituted bythe windings L11 a and L11 b. The first winding Lila has a firstterminal connected to the first line terminal Line+ and a secondterminal coupled to the first POTS/ISDN terminal POTS/ISDN+ through theelliptical filter stages, whilst the second winding L11 b has a firstterminal connected to the second line terminal Line− and a secondterminal coupled to the second POTS/ISDN terminal POTS/ISDN− through theelliptical filter stages. The LC filter cell further comprises a firstcapacitor C11 coupled between the second terminal of its first windingLila and the second terminal of its second winding L11 b.

Each elliptical filter stage comprises a cell with a second symmetricalcoil L12/L13/L14 respectively constituted by the windings L12 a and L12b/L13 a and L13 b/L14 a and L14 b. The first winding L12 a/L13 a/L14 ahas a first terminal coupled to the second terminal of the first windingLila of the above LC filter cell and a second terminal coupled to thefirst POTS/ISDN terminal POTS/ISDN+, whilst the second winding L12 b/L13b/L14 b has a first terminal coupled to the second terminal of thesecond winding L11 b of the LC filter cell and a second terminal coupledto the second POTS/ISDN terminal POTS/ISDN−.

In more detail, the first terminal of the first winding L12 a of thefirst elliptical filter stage is connected to the second terminal of thefirst winding L11 a of the LC filter cell and the first terminal of thesecond winding L12 b of the first elliptical filter stage is connectedto the second terminal of the second winding L11 b of the LC filtercell.

The first terminal of the first winding L13 a of the second ellipticalfilter stage is connected to the second terminal of the first windingL12 a of the first elliptical filter stage and the first terminal of thesecond winding L13 b of the second elliptical filter stage is connectedto the second terminal of the second winding L12 b of the firstelliptical filter stage.

The first terminal of the first winding L14 a of the third ellipticalfilter stage is connected to the second terminal of the first windingL13 a of the second elliptical filter stage and the first terminal ofthe second winding L14 b of the third elliptical filter stage isconnected to the second terminal of the second winding L13 b of thesecond elliptical filter stage.

Finally, the second terminal of the first winding L14 a of the thirdelliptical filter stage is connected to the first POTS/ISDN terminalPOTS/ISDN+, and the second terminal of the second winding L14 b of thethird elliptical filter stage is connected to the second POTS/ISDNterminal POTS/ISDN−.

The 4 symmetrical coils L11 to L14, each with 2 windings (a and b) aretypical RM4 or EP7 size coils.

Each elliptical filter cell further comprises a second capacitorC12/C13/C14 coupled between the second terminal of the first winding L12a/L13 a/L14 a of the second symmetrical coil and the second terminal ofthe second winding L12 b/L13 b/L14 b of the second symmetrical coil, athird capacitor C15/C17/C19 connected across the respective firstwinding L12 a/L13 a/L14 a of the second symmetrical coil and a fourthcapacitor C16/C18/CIA connected across the second winding L2 b/L3 b/L4 bof the second symmetrical coil.

Finally, in some cases, additional resistors are placed in series or inparallel with some of the capacitors for additional damping of the LCfilter stages. For instance, the last elliptical filter cell L14, C19,CIA may have a resistor R11 connected in parallel across the thirdcapacitor C19 and another resistor R12 connected in parallel across thefourth capacitor C1A.

The main idea of the present invention is to reduce the area occupied bythe splitter on a board by replacing the 9^(th) order ISDN or UniversalISDN/POTS splitter low pass filter, known from the prior art and shownat FIG. 1, by a 7^(th) order low pass filter as shown at FIG. 2 or FIG.3.

In the splitter according to the invention and shown at FIG. 2, thisreplacement is achieved by eliminating 1 elliptical filter stage, i.e. 1symmetrical coil RM4 or EP7 type and 3 high voltage capacitors, and byadding 3 small, low cost SMD (Surface Mounted Devices) coils andresistors, as additional series resonance circuits.

By carefully designing this 7^(th) order filter and dimensioning thecomponent values, the following is achieved:

full ETSI standard compliance of the 7^(th) order filter (ETSI TS 101952-1-4), for both 2B1Q and 4B3T ISDN standards, by 1 single filterimplementation;

comparable dynamic performance compared to the 9^(th) order splitterwith respect to transient rejection, noise impact on DSL signals, noiseimpact on ISDN signals; and

smaller size (up to 25% smaller) and lower cost.

A 7^(th) order ISDN or Universal ISDN/POTS splitter low pass filter,consisting of 1 LC filter stage, 2 elliptical filter stages and 3 seriesresonance circuits is schematically represented at FIG. 2.

Between the line terminals Line+ and Line− and the POTS/ISDN terminalsPOTS/ISDN+ and POTS/ISDN−, the 7^(th) order splitter comprises thecascade connection of a LC filter stage L21 a, L21 b, C21, R21, L24, afirst elliptical filter stage L22 a, L22 b, C24, C25, C22, R22, L25 anda second elliptical filter stage L23 a, L23 b, C26, C27, C23, R23, L26.

In more detail, the line terminal Line+ is coupled to the POTS/ISDNterminal POTS/ISDN+ via the series connection of a first winding L21 aof a first symmetrical coil L21 of the LC filter cell, a first windingL22 a of a second symmetrical coil L22 of the first elliptical filtercell, and a first winding L23 a of a third symmetrical coil L23 of thesecond elliptical filter cell.

On the other hand, the line terminal Line− is coupled to the POTS/ISDNterminal POTS/ISDN− via the series connection of a second winding L21 bof the first symmetrical coil L21 of the LC filter cell, a secondwinding L22 b of the second symmetrical coil L22 of the first ellipticalfilter cell, and a second winding L23 b of the third symmetrical coilL23 of the second elliptical filter cell.

The LC filter stage is completed by adding a first damped seriesresonance circuit to the LC filter cell. This first resonance circuit isconstituted by a first capacitor C21 series connected with the parallelconnection of a first resistor R21 and a first coil L24. The firstresonance circuit is connected between the junction point of the firstwinding L21 a of the first symmetrical coil L21 of the LC filter cellwith the first winding L22 a of the second symmetrical coil L22 of thefirst elliptical filter cell and the junction point of the secondwinding L21 b of the first symmetrical coil L21 of the LC filter cellwith the second winding L22 b of the second symmetrical coil L22 of thefirst elliptical filter cell.

The first elliptical filter stage is completed by adding a second dampedseries resonance circuit to the first elliptical filter cell. Thissecond resonance circuit is constituted by a second capacitor C22 seriesconnected with the parallel connection of a second resistor R22 and asecond coil L25. The second resonance circuit is connected between thejunction point of the first winding L22 a of the second symmetrical coilL22 of the first elliptical filter cell with the first winding L23 a ofthe third symmetrical coil L23 of the second elliptical filter cell andthe junction point of the second winding L22 b of the second symmetricalcoil L22 of the first elliptical filter cell with the second winding L23b of the third symmetrical coil L23 of the second elliptical filtercell.

The second elliptical filter stage is completed by adding a third dampedseries resonance circuit to the second elliptical filter cell. Thisthird resonance circuit is constituted by a third capacitor C23 seriesconnected with the parallel connection of a third resistor R23 and athird coil L26. The third resonance circuit is connected between thejunction point of the first winding L23 a of the third symmetrical coilL23 of the second elliptical filter cell with the POTS/ISDN terminalPOTS/ISDN+ and the junction point of the second winding L23 b of thethird symmetrical coil L23 of the second elliptical filter cell with thePOTS/ISDN terminal POTS/ISDN−.

A fourth capacitor C24 is provided in parallel across the first windingL22 a of the second symmetrical coil L22 of the first elliptical filtercell, and a fifth capacitor C25 is provided in parallel across thesecond winding L22 b of this second symmetrical coil L22.

Similarly, a sixth capacitor C26 is provided in parallel across thefirst winding L23 a of the third symmetrical coil L23 of the secondelliptical filter cell, and a seventh capacitor C27 is provided inparallel across the second winding L23 b of this third symmetrical coilL23.

Damping resistors (not shown) may be provided in series or in parallelwith the fourth C24, fifth C25, sixth C26 and/or seventh C27 capacitors.

An additional Common Mode Coil and protector for overvoltages may alsobe provided but are not shown at FIG. 2.

It is to be noted that the symmetrical coils L21, L22 and L23, each withfirst (a) and second (b) windings, are typically of the type EP7 or RM4.

The coils L24, L25 and L26 are small Surface Mounted SMD type coils, oflow values, e.g. a few μH. As there is no DC current going through thesecoils, the size and cost of the coils is relatively small.

The resistors R21, R22, R23 are low-ohmic Surface Mounted SMD type ofresistors, also of small size, as no DC current is flowing through theseresistors.

The architecture of the low pass filter of this 7^(th) orderISDN/Universal splitter results in characteristics that are similar tothose of the above known 9^(th) order filter.

An optimized variant of the 7^(th) order ISDN or Universal ISDN/POTSsplitter low pass filter is shown at FIG. 3. This splitter consists of 1LC filter stage, 2 elliptical filter stages and 2 series resonancecircuits.

In this more optimized embodiment of the 7^(th) order ISDN or ISDN/POTSUniversal splitter, only 2 series resonance circuits C31, R31, L34 andC32, R32, L35 are used. In other words, with respect to the embodimentshown at FIG. 2, the second elliptical filter cell is no longerassociated with the third damped series resonance circuit comprising thethird capacitor C23 series connected with the parallel connection of thethird resistor R23 and the third coil L26, but only with third capacitorC33 as shown at FIG. 3. This third capacitor C33 is thus directlyconnected between the junction point of the first winding L33 a of thethird symmetrical coil L33 of the second elliptical filter cell with thePOTS/ISDN terminal POTS/ISDN+ and the junction point of the secondwinding L33 b of the third symmetrical coil L33 of the second ellipticalfilter cell with the POTS/ISDN terminal POTS/ISDN−.

By correct dimensioning the component values, also with this filtershown at FIG. 3, full compliancy to ETSI standard ETSI TS 101 952-1-4 isguaranteed and a dynamic performance (transient rejection, noise impactof ISDN on xDSL signals, noise impact of xDSL on ISDN signals),comparable to the known 9^(th) order splitter is achieved.

Compared to the embodiment shown at FIG. 2, this more optimizedembodiment shown at FIG. 3 requires even less space and has a bettercost advantage.

FIG. 4 shows the size comparison of the 7^(th) order splitter (right: 33mm×17 mm=5.61 cm²) versus the 9^(th) order splitter (left: 42 mm×17mm=7.14 cm²) showing more than 25% size reduction with the 7^(th) ordersplitter. Only the top side components are shown at FIG. 4, not the SMDcomponents at the bottom side, L representing coils and C representingcapacitors.

It is to be noted that other implementation of the 7^(th) order ISDN orISDN/POTS/Universal splitter low pass filter, with e.g. series resistorsfor the elliptical caps, other positions of series resonance circuits,etc. may be derived from the basic architectures shown at FIG. 2 andFIG. 3.

Derived implementations of the 7^(th) order ISDN/POTS Universal splitterlow pass filter are for instance:

implementations with 1, 2 or 3 series resonance circuits;

different positions for 1 or 2 series resonance circuits, e.g. togetherwith C31, C32 or C33 in case of 1 series resonance circuit, togetherwith C31 and C32 (as shown at FIG. 3), C31 and C33 or C32 and C33 incase of 2 series resonance circuits; and

use of series or parallel damping resistors on the fourth C24, fifthC25, sixth C26 and/or seventh C27 capacitors as mentioned above withrespect to FIG. 2.

A final remark is that embodiments of the present invention aredescribed above in terms of functional blocks. From the functionaldescription of these blocks, given above, it will be apparent for aperson skilled in the art of designing electronic devices howembodiments of these blocks can be manufactured with well-knownelectronic components. A detailed architecture of the contents of thefunctional blocks hence is not given.

While the principles of the invention have been described above inconnection with specific apparatus, it is to be clearly understood thatthis description is merely made by way of example and not as alimitation on the scope of the invention, as defined in the appendedclaims.

1. A splitter for an ISDN/POTS telecommunication system, said splitterhaving first and second line terminals and first and second POTS/ISDNterminals, and comprising the cascade connection of a LC filter cell anda plurality of elliptical filter cells, said LC filter cell comprising afirst symmetrical coil having a first winding with a first terminalconnected to said first line terminal and with a second terminal coupledto said first POTS/ISDN terminal via said plurality of elliptical filtercells, and having a second winding with a first terminal connected tosaid second line terminal and with a second terminal coupled to saidsecond POTS/ISDN terminal via said plurality of elliptical filter cells,said LC filter cell further comprising a first capacitor coupled betweenthe second terminal of the first winding of said first symmetrical coiland the second terminal of the second winding of said first symmetricalcoil, each elliptical filter cell of said plurality comprising a secondsymmetrical coil having a first winding with a first terminal coupled tothe second terminal of the first winding of said LC filter cell and witha second terminal coupled to said first POTS/ISDN terminal and having asecond winding with a first terminal coupled to the second terminal ofthe second winding of said LC filter cell and with a second terminalcoupled to said second POTS/ISDN terminal, each elliptical filter cellfurther comprising a second capacitor coupled between the secondterminal of the first winding of the second symmetrical coil of saidelliptical filter cell and the second terminal of the second winding ofsaid second symmetrical coil, a third capacitor connected across thefirst winding of said second symmetrical coil and a fourth capacitorconnected across the second winding (L22 b; L23 b) of said secondsymmetrical coil, wherein said splitter comprises at least one RLcircuit constituted by the parallel coupling of a resistor and a coiland connected in series with the second capacitor of an ellipticalfilter cell of said plurality of elliptical cells.
 2. The splitteraccording to claim 1, wherein said splitter comprises another RL circuitconstituted by the parallel coupling of a second resistor and a secondcoil and connected in series with the first capacitor of said LC filtercell between the second terminal of the first winding of said firstsymmetrical coil of said LC filter and the second terminal of the secondwinding of said first symmetrical coil of said LC filter.
 3. Thesplitter according to claim 1 claim 1, wherein said splitter comprisesthe series connection of said LC filter cell, a first elliptical filtercell comprising a first second capacitor and a second elliptical filtercell comprising a second second capacitor, wherein the first secondcapacitor of said first elliptical filter cell is connected in serieswith a first RL circuit constituted by the parallel coupling of a firstresistor and a first coil between the second terminal of the firstwinding of the second symmetrical coil of said first elliptical filtercell and the second terminal of the second winding of the secondsymmetrical coil of said first elliptical filter cell, wherein thesecond capacitor of said second elliptical filter cell is connected inseries with a second RL circuit constituted by the parallel coupling ofa second resistor and a second coil between the second terminal of thefirst winding of the second symmetrical coil of said second ellipticalfilter cell and the second terminal of the second winding of the secondsymmetrical coil of said second elliptical filter cell.
 4. The splitteraccording to claim 1, wherein said splitter further comprises a firstdamping resistor connected in parallel with the third capacitor acrossthe first winding of the second symmetrical coil of said ellipticalfilter cells, and a second damping resistor connected in parallel withthe fourth capacitor across the second winding of said secondsymmetrical coil.
 5. The splitter according to claim 1, wherein saidsplitter further comprises a third damping resistor connected in serieswith the third capacitor across the first winding of the secondsymmetrical coil of said elliptical filter cells, and a fourth dampingresistor connected in series with the fourth capacitor across the secondwinding of said second symmetrical coil.